It is sought more particularly here below in this document to describe problems existing in the field of drilling pipes. The invention of course is not limited to this particular field of application, but is of interest for any technique that implements a structure comprising at least one pair of successive pipes through which it is intended to transmit data.
For illustrative purposes, FIG. 1 depicts a drilling installation 1 comprising a string of drilling tools 10 that convey drilling fluid and rotational power from the top-drive system 11 (denoted hereafter TDS) on the surface down to the bottom-hole assembly 12 (denoted hereafter BHA) and drill bit underground. Most of these tools are drill pipes of length typically 10 m, which are mechanically connected to each other by pin and box threaded end sections. As the total distance between the BHA 12 and the TDS 11 at the surface can be in excess of 3 km, there can be as many as 300 separate sections of drill pipe. Mud and other drilling fluids pass through the centre of the pipes, where operating temperatures can reach more than 200° C.
In the underground drilling industry, high-speed, bi-directional data communications are required between the TDS 11 and the BHA 12. High-speed data communication of this type can substantially increase the efficiency of the drilling rig by facilitating much improved “Measurement While Drilling (MWD)”, “Logging While Drilling (LWD)” and “Directional Drilling (DD)” whilst reducing non-productive time. Real-time, rapid data collection (such as temperature, pressure, salinity, etc.) in the uplink direction and corresponding control data sent in the downlink direction enable optimisation of the drilling process for more accurate well placement and extended reach without the need for a suspension of operations. Unfortunately, such is the structure of the drilling installation and the environmental conditions in which it must operate that reliable, fast, bi-directional data communications are very difficult to provide.
A first known data communication system for use in a drill string is based on a mud pulse technique. This technique consists of sending sonic pulses up and down the drilling fluid in the pipes. The chief drawbacks to this mud pulse system are that the data rate is slow, i.e. less than 10 baud, the system is complex and expensive, the results can be inconsistent, and the range of performance can be limited. Thus, this first known system tends to be used only as an emergency back-up when all other means of communication have failed.
To achieve much higher data rates, it is known in the state of the art to use a transmission line inserted into the string of drill pipes to make a connection between an electronic data interface 110 at the surface of the rig and a downhole interface 120 underground. However, given the necessity of using separate sections of pipe and the way in which they must be assembled on site, the transmission line cannot be a single length of line but must also be a series of separate sections whose length is roughly equal to that of the drill pipes and whose ends must be coupled together by a coupling device (or coupler) to ensure reliable data communications along the entire length of the link, at all times and under all operating conditions.
The electrical losses along the length of the data link in the drill string must be kept within manageable limits, so as to ensure that the transmitted signal from one end reaches the receiver at the other end with an adequate signal-to-noise ratio. Due to the length of the drill string, the individual drill pipes (called Wired Drill Pipes or WDPs) are assembled into groups 130, each group of which interfaces to its neighbor via a repeater 140. The role of the repeater is to detect and demodulate the incoming signals arriving from both the uplink and downlink directions, remove unwanted noise, regenerate and amplify the required signals and pass them onto the next group of pipes and repeater. As illustrated in FIG. 1, there will be several of these pipe groups 130 and repeaters 140, the exact number depending upon the power budget for the whole link. However, the number of repeaters must be kept as low as possible to minimise both the cost of installation and maintenance. The repeaters are generally battery powered. Hence, it is essential that the losses in both the sections of the passive transmission line and the couplers are kept as low as is technically feasible.
Different data communication systems using transmission line and couplers have been proposed. In this regard, pipes equipped with electrical paths such as those described in the documents US-2004/0177956, US 2008/0041575 or even U.S. Pat. No. 2,379,800 are known in the prior art.
A second known data communication system uses a transmission line passing down the length of the drill string whereby the separate sections of transmission line are connected together with specially-designed electrical connectors. This second system suffered badly from the lack of reliability in the electrical connections due to the presence of oil, grease, water, sand and other abrasive materials at the site of the joint.
A third known system is presented in the patent document U.S. Pat. No. 6,670,880 B 1. This system uses couplers based on magnetic coupling. Each pipe comprises a passage formed in the pipe wall for enabling the passage of a transmission line. Embedded in the shoulders of each end of a pipe is a hollow ring of electrically insulating, magnetic material, inside of which is located a coil of wire. In operation, i.e. when the ends of first and second pipes come together, a varying current applied to the first coil of the first pipe generates a varying magnetic field in the first electrically insulating, magnetic ring of the first pipe, which varying magnetic field is conducted to and thereby produces a varying magnetic field in the second electrically insulating, magnetic ring of the second pipe, which magnetic field thereby generates a varying electrical current in the second coil of the connected second pipe. The coupling is substantially independent of the angular orientation of the two pipes and is not reliant on any electrical connection. This system permits reliable, bi-directional data transmission along the drill string at data rates of about 50 kbits/s. Higher data rates however are difficult to achieve owing to the use of magnetic materials and the relatively high inductance of the coupled coils.
To make the further advances in drilling performance now being demanded by industry, the data rates must be increased towards a new target value of about 50 Mb/s.